Massive embarrasment for the Queen and her racehorse trainer Sir Michael Stoute today as one of her prize steeds - the 2013 Ascot Gold Cup winning filly Estimate - tested positive for morphine in random tests conducted by Britain's horseracing authorities.

Keen-witted royalists wil remember that this was the win that caused the Queen to break into an uncharacteristic show of emotion, laughing and slapping her knees as it romped home.

The shock news will cast an unfortunate pall over a day which was otherwise to be notable for the birthday celebrations for Prince George, which were taking place at Kensington Palace today.

The horse was one of five animals to be found to have morphine in its blood in recent tests, but it seems unlikely that this story will develop into a full-blown royal doping scandal.

Horse racing experts say that morphine can occur in some food products, which may be how the substance found its way into the thoroughbred's system. The Queen's bloodstock adviser indicated in a statement that this is the suspected source of the morphine.

While this will be some comfort to the Queen, she will likely still be livid at the news.

The Queen is passionate about racing, and the royal stud is her greatest pride and joy. The idea that her horses are being given dodgy feed will not go down well.

In a statement John Warren, the Queen's bloodstock and racing advisor, said: "On Thursday July 17 the British Horseracing Authority announced that a number of post-race samples, obtained from recent race meetings, had been found to indicate the presence of morphine, which is a prohibited substance on race days. Five horses, under the care of various trainers, were affected.

"I can confirm that one of those horses was Estimate, the five year-old filly trained by Sir Michael Stoute and owned by The Queen. Initial indications are that the positive test resulted from the consumption of a contaminated feed product.

"Sir Michael is working closely with the feed company involved to discover how the product may have become contaminated prior to delivery to his stables.

"As the BHA investigates this matter, including potential links between the different cases, Sir Michael continues to offer his full co-operation. There will be no further comment until the BHA announces its considered findings. Her Majesty has been informed of the situation."